1) Field of the Invention
The present invention relates to apparatuses and methods for measuring a tubular member and, more specifically, for measuring one or more cross-sectional characteristics of the tubular member.
2) Description of Related Art
Tubular members such as pipes, ducts, and the like are used in various applications, for example, as transport passageways through which gases can be delivered for heating, ventilation, and air conditioning. In other applications, pipes, ducts, and other tubular members can also be used to deliver liquids, such as water, waste liquids, and the like. In the aircraft industry, tubular members are provided as ducts in environmental control systems (ECS) of modern commercial aircraft. An ECS and, hence, the ducts therein, provides air throughout the cabin of the aircraft in order to provide air as required for the respiratory needs of the occupants, to clear contaminants and odors from the cabin air, to control the temperature and humidity of the cabin environment, and to provide adequate cabin pressure. Air can also be delivered throughout the aircraft for de-icing of the aircraft, for heating of the cargo area of the aircraft, for pneumatic systems, and for cooling hydraulic and electrical systems.
The tubular members used for ducts, such as for the ECS of an aircraft, are typically formed of composite or metallic materials. Common composite reinforcement materials include fiberglass (BMS 8-80, 8-139, or 8-226), graphite (BMS 8-168), and Kevlar® (BMS 8-264), a registered trademark of E.I. du Pont de Nemours and Company. These reinforcement materials, which can be provided as sheets, fibers, or the like, can be preimpregnated with epoxy or polyester resin, which hardens when subjected to heat and pressure. Alternatively, the reinforcement materials can be reinforced with thermoplastic materials such as polyetherimide (PEI), which is available under the trade name Ultem®, a registered trademark of General Electric Company. In either case, ducts formed of composite materials can be lightweight and strong. Alternatively, the tubular members can be relatively thin-walled structures that are formed of metals such as steel.
Tubular members, such as composite ducts, are typically dimensionally tested, e.g., by measuring the circumference or diameter of the member to see if the cross-sectional size of the member matches the desired size. For example, the inner or outer diameter of the tubular member can be measured using a gauge, calipers, micrometers, and the like. However, due to the flexible nature of composite materials and many thin-walled structures, the tubular members can flex during testing, thereby reducing the accuracy of the measurement. Alternatively, the tubular member can be measured by extending a flexible measuring tape around the circumference of the member. In the case of a “pi tape,” the measurement units marked on the tape are adjusted by the value of the pi constant, i.e., the marked units reflect the linear length of the tape divided by pi. Thus, an operator can quickly determine the diameter of the tubular member by extending the tape around the tubular member and reading the diameter from the circumferential markings on the tape. However, regardless of the units provided on the measuring tape, the measurement made therewith is dependent on the placement of the tape and the reading that is taken by the operator. Thus, like the other mechanical measuring devices listed above, the accuracy of the tape is limited by the ability of the operator.
In yet another conventional method of measurement, an electronic coordinate measuring machine is used to determine relative coordinate positions for several points around the circumferential perimeter of the tubular member. The coordinate positions are then used to determine the cross-sectional size of the member, e.g., by using a “best-fit” technique in which the coordinate positions are matched up with an equation describing a best-fit circle, and a circumference of the member is thereby approximated. The coordinate measuring machine can be capable of very accurately determining the positions of the several points, but the circumference determined by the machine is only an approximate measurement. In particular, if the cross-sectional shape of the member is not circular, the best-fit circle will not accurately correspond to the cross-sectional shape of the member, and the accuracy of the dimensions determined with the machine can be decreased.
Thus, there exists a need for an apparatus and method for accurately measuring the cross-sectional characteristics of a tubular member formed of a thin-walled or otherwise flexible material. The apparatus should be capable of accurately determining at least one characteristic, such as the diameter or wall thickness, of the tubular member, even if the cross-sectional shape of the member does not define a perfect or uniform polygonal shape such as a circle.